U.S. patent application number 14/469708 was filed with the patent office on 2016-03-03 for feedback for electronic pre-distortion in an optical transmitter.
The applicant listed for this patent is Alcatel-Lucent USA Inc.. Invention is credited to Andrew Adamiecki, Gregory Raybon, Chandrasekhar Sethumadhavan, Peter J. Winzer.
Application Number | 20160065311 14/469708 |
Document ID | / |
Family ID | 54065478 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160065311 |
Kind Code |
A1 |
Winzer; Peter J. ; et
al. |
March 3, 2016 |
FEEDBACK FOR ELECTRONIC PRE-DISTORTION IN AN OPTICAL
TRANSMITTER
Abstract
We disclose an optical transponder, in which one or more
all-electronic feedback paths are used to obtain a relatively
accurate estimate of the device-specific signal distortions in the
transmitter portion thereof. The obtained estimate is used to
enable the digital signal processor of the optical transponder to
carry out electronic pre-distortion (EPD) that can significantly
reduce or cancel these signal distortions without the use of
detailed factory-calibration measurements or optics dedicated to
feedback purposes. The use of all-electronic feedback paths may
enable a beneficial reduction in the cost of the EPD functionality,
e.g., by eliminating a significant extra cost associated with the
implementation of optically generated feedback.
Inventors: |
Winzer; Peter J.; (Aberdeen,
NJ) ; Adamiecki; Andrew; (Morganville, NJ) ;
Sethumadhavan; Chandrasekhar; (Old Bridge, NJ) ;
Raybon; Gregory; (Shrewsbury, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alcatel-Lucent USA Inc. |
Murray Hill |
NJ |
US |
|
|
Family ID: |
54065478 |
Appl. No.: |
14/469708 |
Filed: |
August 27, 2014 |
Current U.S.
Class: |
398/193 |
Current CPC
Class: |
H04B 10/588 20130101;
H04B 10/564 20130101; H04B 10/25891 20200501; H04B 10/58
20130101 |
International
Class: |
H04B 10/58 20060101
H04B010/58; H04B 10/25 20060101 H04B010/25; H04B 10/564 20060101
H04B010/564 |
Claims
1. An apparatus comprising: an optical transmitter configured to
generate a modulated optical signal based on an electrical digital
signal; a digital signal processor configured to apply electronic
pre-distortion to generate the electrical digital signal in a
manner that reduces an amount of signal distortion in the modulated
optical signal caused by hardware of the optical transmitter; and a
first all-electronic feedback path between the optical transmitter
and the digital signal processor configured to provide a first
feedback signal for the digital signal processor to determine one
or more parameter values for the electronic pre-distortion.
2. The apparatus of claim 1, wherein the electronic pre-distortion
is configured to cause an effective signal-transfer function
exhibited by the optical transmitter to differ from a
signal-transfer function imposed by the hardware of the optical
transmitter.
3. The apparatus of claim 1, further comprising a second
all-electronic feedback path between the optical transmitter and
the digital signal processor configured to provide a second
feedback signal for the digital signal processor to determine the
one or more parameter values for the electronic pre-distortion.
4. The apparatus of claim 3, further comprising a first partially
optical feedback path between the optical transmitter and the
digital signal processor configured to provide a third feedback
signal for the digital signal processor to determine the one or
more parameter values for the electronic pre-distortion.
5. The apparatus of claim 4, further comprising a second partially
optical feedback path between the optical transmitter and the
digital signal processor configured to provide a fourth feedback
signal for the digital signal processor to determine the one or
more parameter values for the electronic pre-distortion.
6. The apparatus of claim 1, further comprising a first partially
optical feedback path between the optical transmitter and the
digital signal processor configured to provide a second feedback
signal for the digital signal processor to determine the one or
more parameter values for the electronic pre-distortion.
7. The apparatus of claim 6, wherein the first partially optical
feedback path comprises: an optical tap configured to tap out a
portion of an optical power of the modulated optical signal; an
optical-to-electrical converter configured to convert the portion
of the optical power of the modulated optical signal into a
corresponding electrical signal; and an electrical conductor
configured to direct the corresponding electrical signal to an
input interface of the digital signal processor to provide thereat
the second feedback signal.
8. The apparatus of claim 7, wherein the optical-to-electrical
converter comprises a photodiode.
9. The apparatus of claim 7, wherein the optical-to-electrical
converter comprises a coherent optical detector coupled to an
optical local-oscillator source.
10. The apparatus of claim 1, wherein the optical transmitter
comprises an optical modulator; and wherein the first
all-electronic feedback path comprises: an electrical pick-up
terminal located in an electrical-circuit portion of the optical
modulator; and an electrical conductor configured to direct an
electrical signal picked-up by the electrical pick-up terminal to
an input interface of the digital signal processor to provide
thereat the first feedback signal.
11. The apparatus of claim 10, wherein the optical modulator
comprises: an optical waveguide; a first electrode positioned along
a length of the optical waveguide and having a first end and a
second end, wherein: the first end is configured to receive an
electrical drive signal generated based on the electrical digital
signal; and the electrical pick-up terminal is electrically
connected to the second end.
12. The apparatus of claim 11, wherein the optical transmitter is
configured to generate the electrical drive signal in a manner that
causes the first electrode to operate as a traveling-wave
electrode.
13. The apparatus of claim 11, further comprising a second
all-electronic feedback path between the optical transmitter and
the digital signal processor configured to provide a second
feedback signal for the digital signal processor to determine the
one or more parameter values for the electronic pre-distortion,
wherein the second all-electronic feedback path comprises: an
additional electrical pick-up terminal; and an additional
electrical conductor configured to direct an electrical signal
picked-up by the additional electrical pick-up terminal to the
input interface of the digital signal processor to provide thereat
the second feedback signal.
14. The apparatus of claim 13, wherein the optical modulator
further comprises a second electrode positioned along a length of
the optical waveguide; and wherein the additional electrical
pick-up terminal is electrically connected to the second
electrode.
15. The apparatus of claim 13, wherein: the optical transmitter
comprises an additional optical modulator; the additional optical
modulator comprises a second electrode positioned along a length of
another optical waveguide; and the additional electrical pick-up
terminal is electrically connected to the second electrode.
16. The apparatus of claim 1, wherein the first all-electronic
feedback path comprises an analog-to-digital converter.
17. An electronic pre-distortion method comprising: configuring an
optical transmitter to generate an optical output signal carrying a
training data sequence; receiving, via a first all-electronic
feedback path, a first feedback signal corresponding to the optical
output signal, wherein the first all-electronic feedback path is
configured to electrically connect the optical transmitter and a
digital signal processor; determining one or more parameter values
for electronic pre-distortion based on the training data sequence
and the first feedback signal; and configuring the optical
transmitter to generate an optical signal carrying payload data
with the digital signal processor being configured to apply the
electronic pre-distortion to the payload data using the one or more
parameter values.
18. The method of claim 17, further comprising receiving, via a
second all-electronic feedback path, a second feedback signal,
wherein the second all-electronic feedback path is configured to
electrically connect the optical transmitter and the digital signal
processor; and wherein the step of determining comprises
determining the one or more parameter values for the electronic
pre-distortion based on the second feedback signal.
19. The method of claim 17, further comprising receiving, via a
first partially optical feedback path, a second feedback signal,
wherein the first partially optical feedback path is configured to
electrically connect the optical transmitter and the digital signal
processor; and wherein the step of determining comprises
determining the one or more parameter values for the electronic
pre-distortion based on the second feedback signal.
20. The method of claim 17, wherein the step of determining
comprises: estimating, based on first feedback signal, a frequency
response of the optical transmitter at an optical output thereof;
and determining the one or more parameter values for the electronic
pre-distortion based on the estimated frequency response.
Description
BACKGROUND
[0001] 1. Field
[0002] The present disclosure relates to optical communication
equipment and, more specifically but not exclusively, to the
generation of feedback signals for electronic pre-distortion in an
optical transmitter.
[0003] 2. Description of the Related Art
[0004] This section introduces aspects that may help facilitate a
better understanding of the disclosure. Accordingly, the statements
of this section are to be read in this light and are not to be
understood as admissions about what is in the prior art or what is
not in the prior art.
[0005] Optical transponders are critical elements within optical
transport systems. A continued trend in the development of optical
transponders is towards higher integration and higher modulation
speeds. However, the use of a relatively high modulation speed
tends to exacerbate to a significant degree the performance penalty
caused by non-optimal frequency characteristics of certain
integrated components of the optical transponder.
SUMMARY OF SOME SPECIFIC EMBODIMENTS
[0006] Disclosed herein are various embodiments of an optical
transponder, in which one or more all-electronic feedback paths are
used to obtain a relatively accurate estimate of the
device-specific signal distortions in the transmitter portion
thereof. The obtained estimate is used to enable the digital signal
processor of the optical transponder to carry out electronic
pre-distortion (EPD) that can significantly reduce or cancel these
device-specific signal distortions without the use of detailed
factory-calibration measurements or optics dedicated to feedback
purposes. The use of all-electronic feedback paths may enable a
beneficial reduction in the cost of the EPD functionality, e.g., by
eliminating a significant extra cost associated with the
implementation of optically generated feedback.
[0007] In some embodiments, the one or more all-electronic feedback
paths may be used in parallel with one or more partially optical
feedback paths.
[0008] According to one embodiment, provided is an apparatus
comprising: an optical transmitter configured to generate a
modulated optical signal based on an electrical digital signal; a
digital signal processor configured to apply electronic
pre-distortion to generate the electrical digital signal in a
manner that reduces an amount of signal distortion in the modulated
optical signal caused by hardware of the optical transmitter; and a
first all-electronic feedback path between the optical transmitter
and the digital signal processor configured to provide a first
feedback signal for the digital signal processor to determine one
or more parameter values for the electronic pre-distortion.
[0009] According to another embodiment, provided is an electronic
pre-distortion method comprising the steps of: (A) configuring an
optical transmitter to generate an optical output signal carrying a
training data sequence; (B) receiving, via a first all-electronic
feedback path, a first feedback signal, wherein the first
all-electronic feedback path is configured to electrically connect
the optical transmitter and a digital signal processor; (C)
determining one or more parameter values for electronic
pre-distortion based on the first feedback signal; and (D)
configuring the optical transmitter to generate an optical signal
carrying payload data with the digital signal processor being
configured to apply the electronic pre-distortion to the payload
data using the one or more parameter values.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Other aspects, features, and benefits of various disclosed
embodiments will become more fully apparent, by way of example,
from the following detailed description and the accompanying
drawings, in which:
[0011] FIG. 1 shows a block diagram of an optical transponder
according to an embodiment of the disclosure;
[0012] FIG. 2 shows a block diagram of an electro-optic circuit
that can be used in the transponder of FIG. 1 according to an
embodiment of the disclosure;
[0013] FIG. 3 schematically shows conventional radio-frequency
termination of traveling-wave electrodes in a Mach-Zehnder
modulator;
[0014] FIG. 4 shows a flowchart of an electronic pre-distortion
method that can be used in the transponder of FIG. 1 according to
an embodiment of the disclosure;
[0015] FIG. 5 graphically illustrates the use of all-electronic
feedback paths in the method of FIG. 4 according to an embodiment
of the disclosure; and
[0016] FIG. 6 shows a block diagram of an optical transponder
according to an alternative embodiment of the disclosure.
DETAILED DESCRIPTION
[0017] Modern high-speed optical transponders may use digital pulse
shaping (such as Nyquist pulse shaping, root-raised-cosine pulse
shaping, etc.) and/or electronic pre-distortion (EPD) to compensate
the deleterious effects of certain fundamental or spurious
transmit-side hardware impairments. For example, one of the
fundamental transmit-side hardware impairments may be related to
the inherent nonlinear (e.g., sinusoidal) transfer characteristics
of an optical (e.g., Mach-Zehnder) modulator. It is anticipated
that accurate compensation of hardware impairments will play an
increasingly critical role in the design and operation of highly
integrated low-cost optical transponders capable of meeting
high-quality performance standards.
[0018] In some prior-art optical transponders, no feedback at all
is used to control the EPD in the transmitter. In some of these
cases, the digital signal processor (DSP) of the optical
transponder is configured to modify the drive signal(s) applied to
the optical modulator in the transmitter essentially based on a
best-guess estimate of the hardware imperfections and/or
impairments, e.g. associated with digital-to-analog converters
(DACs), driver amplifiers, and optical modulators.
Disadvantageously, the best-guess estimate may not be accurate
enough to avoid unacceptably large performance penalties due to
significant differences between nominally identical optical
transmitters of the same model, e.g., caused by fabrication-process
variances, device aging, etc.
[0019] Some optical transponders may be configured to use some form
of optical feedback to control the EPD in the transmitter. For
example, the feedback generation may include (i) tapping the
optical output signal generated by the transmitter, (ii) converting
the tapped optical signal into a corresponding electrical signal
using a dedicated photo-detector or optical receiver, and (iii)
feeding a digital version of this electrical signal to the DSP for
setting the parameters of and controlling the EPD implemented
therein. While this optical feedback scheme may overcome some of
the above-indicated drawbacks of the best-guess approach, it also
incurs the extra cost of a photo-detector or an optical receiver
dedicated exclusively to feedback purposes in the corresponding
optical transponder.
[0020] At least some of the above-indicated problems in the state
of the art are addressed by various embodiments of a disclosed
optical transponder, in which an all-electronic feedback path
enables the DSP to have access to a large portion of the
device-specific information on hardware-induced signal distortions,
thereby enabling the DSP to implement the EPD capable of accurately
compensating the deleterious effects of hardware imperfections
and/or impairments without dedicated optics or detailed factory
calibration measurements. In some embodiments, the all-electronic
feedback path may be used in parallel and in combination with an
optical feedback path. Various embodiments of an optical
transponder that incorporates an all-electronic feedback path, with
or without an additional optical feedback path, are described in
more detail below in reference to FIGS. 1-6.
[0021] FIG. 1 shows a block diagram of an optical transponder 100
according to an embodiment of the disclosure. Transponder 100 is
designed for bidirectional data transmission over an optical
transport link, e.g., including an optical fiber or a fiber-optic
cable (not explicitly shown in FIG. 1). As such, transponder 100
includes a receiver portion and a transmitter portion. In an
example embodiment, the receiver portion of transponder 100
comprises a coherent optical receiver 160, including an optical
local-oscillator (LO) source 164. The transmitter portion of
transponder 100 comprises an optical transmitter 170, including a
laser 172 and an optical modulator 176.
[0022] In operation, receiver 160 receives an optical input signal
190 from the optical transport link as indicated in FIG. 1. Optical
LO source 164 generates an optical reference signal having an
optical-carrier frequency (wavelength) that is sufficiently close
to that of optical signal 190 to enable coherent (e.g., intradyne
or homodyne) detection of the latter signal. For this purpose,
optical LO source 164 may include a relatively stable tunable laser
whose output wavelength is approximately the same as the carrier
wavelength of optical signal 190. Receiver 160 optically mixes
optical signal 190 and the reference signal generated by optical LO
source 164 to generate a plurality of mixed optical signals (not
explicitly shown in FIG. 1). An optical-to-electrical (O/E)
converter of receiver 160 then converts the mixed optical signals
into a corresponding plurality of electrical signals, e.g.,
indicative of complex values corresponding to two
orthogonal-polarization components of signal 190. For example, a
first pair of the electrical signals may be an analog in-phase
signal and an analog quadrature signal, respectively, corresponding
to a first (e.g., x) polarization component of signal 190. A second
pair of the electrical signals may similarly be an analog in-phase
signal and an analog quadrature signal, respectively, corresponding
to a second (e.g., y) polarization component of signal 190.
[0023] Each of the electrical signals generated by receiver 160 is
optionally amplified in a respective one of electrical amplifiers
166 and converted into a digital form 128 in a corresponding one of
a plurality of analog-to-digital converters (ADCs) 130. Electrical
digital signals 128 produced by ADCs 130 are then processed by a
DSP 170, e.g., as known in the art, to recover the data of a data
stream 118 encoded in optical signal 190. A serializer/deserializer
(SERDES) 110 operates to appropriately de-serialize data stream 118
into a plurality of sub-streams 104 for distribution to the clients
of transponder 100 over the data plane of the corresponding
communication system (not explicitly shown in FIG. 1).
[0024] Transmitter 170 operates to generate an optical output
signal 188 for transmission over the optical transport link in a
manner that causes optical signal 188 to carry data received by
transponder 100, via a plurality of sub-streams 102, from the data
plane of the communication system. SERDES 110 is configured to
appropriately serialize sub-streams 102 to generate a data stream
112 that is then encoded in optical signal 188. More specifically,
DSP 120 processes data stream 112 to generate a plurality of
electrical digital signals 124. Each of electrical digital signals
124 is converted into an analog form in a corresponding one of a
plurality of DACs 140, and the resulting electrical analog signals
are amplified and biased, using electrical amplifiers 146, to
generate drive signals for driving optical modulator 176. Using the
drive signals, optical modulator 176 then modulates a CW optical
beam 174 received from laser 172 to generate optical signal
188.
[0025] In an example embodiment, DSP 120 uses a suitable EPD method
to generate electrical digital signals 124 in a manner that tends
to cancel signal distortions caused or imposed by some of the
downstream circuits in transponder 100, e.g., DACs 140, electrical
amplifiers 146, and optical modulator 176. An example embodiment of
the EPD method that can be used in DSP 120 is described in more
detail below in reference to FIG. 4. Briefly, the EPD method used
in DSP 120 may operate based on a plurality of feedback signals 154
provided to the DSP via a plurality of all-electronic feedback
paths 180 (labeled 180.sub.1-180.sub.n, where n is a positive
integer greater than one). As used herein, the term
"all-electronic" means that feedback paths 180 do not use or have
any optical-to-electrical (O/E) or electrical-to-optical (E/O)
converters therein. This feature characterizes both terminal
portions of each feedback path 180.sub.i (where i=1, . . . n) and
its middle portion.
[0026] For example, an i-th feedback path 180.sub.i may have an
electrical pick-up terminal located in the electrical-circuit
portion of optical modulator 176. An electrical signal picked-up by
or applied to the electrical pick-up terminal of feedback path
180.sub.i may propagate along an electrical conductor to an input
port of one of (optional) electrical amplifiers 148. A
corresponding amplified electrical signal generated by electrical
amplifier 148 is then converted into digital form in the
corresponding one of ADCs 150 to generate a respective one of
feedback signals 154. Note that the feedback signal remains in the
electrical domain all the way from its pick-up point at the first
end terminal of feedback path 180.sub.i located in the
electrical-circuit portion of optical modulator 176 to its drop-off
point at the second end terminal of feedback path 180.sub.i
connected to the input interface of DSP 120. No portion of feedback
signal 154 is generated using an optically tapped portion of the
power of optical signal 188.
[0027] FIG. 2 shows a block diagram of an electro-optic circuit 200
that can be used in transponder 100 (FIG. 1) according to an
embodiment of the disclosure. Circuit 200 includes a Mach-Zehnder
modulator 210 that can be a part of optical modulator 176 (FIG. 1).
In some embodiments, optical modulator 176 may include two or more
modulators similar to modulator 210. In different embodiments,
these two or more modulators may be arranged in a nested
configuration, be connected to each other in a serial or parallel
configuration, or be optically unconnected to one another.
[0028] Modulator 210 comprises two waveguide arms, labeled
212.sub.1 and 212.sub.2, and electrodes 214 and 216. Electrode 214
is positioned over waveguide arm 212.sub.1, as indicated in FIG. 2.
Electrode 216 is positioned between waveguide arms 212.sub.1 and
212.sub.2, as further indicated in FIG. 2.
[0029] An electrical amplifier 220 is configured to drive modulator
220 by applying a radio-frequency (RF) drive signal 218 to
electrodes 214 and 216. Drive signal 218 is generated based on an
electrical digital signal 234 received by circuit 200 from the
corresponding DSP, e.g., DSP 120 of FIG. 1, by first converting
electrical digital signal 234 into a corresponding electrical
analog signal in a DAC 230, and then amplifying and biasing this
analog signal in amplifier 220. In an example embodiment, DAC 230
can be one of DACs 140 (FIG. 1), and amplifier 220 can be one of
amplifiers 146 (FIG. 1).
[0030] When the modulation speed is relatively high, electrodes 214
and 216 are conventionally designed to operate as traveling-wave
electrodes. As known in the art, one difference between a lumped
electrode and a traveling-wave electrode is that, for the latter,
the electrode length along the waveguide arm is not small compared
to the wavelength of the corresponding RF drive signal, such as
signal 218. As a result, electrodes 214 and 216 function as an
extension of the driving transmission line that delivers signal 218
to the electrodes and are typically designed to approximately match
the impedance of that transmission line. The driving transmission
line is electrically connected to apply signal 218 to the first
(e.g., left in FIG. 2) ends of electrodes 214 and 216. The second
(e.g., right in FIG. 2) ends of electrodes 214 and 216 are
connected to another transmission line, labeled 240, whose
impedance is similarly matched to the impedance of the electrodes.
Transmission line 240 is connected to an ADC 250, which is further
connected to the DSP as indicated in FIG. 2. In an example
embodiment, transmission line 240 can be a part of one of feedback
paths 180 (FIG. 1), and ADC 250 can be one of ADCs 150 (FIG. 1).
The points of connection of transmission line 240 to electrodes 214
and 216 can serve as a pick-up terminal for the feedback path
180.
[0031] FIG. 3 schematically shows a conventional RF termination of
traveling-wave electrodes 314 and 316, which can be compared and
contrasted with the RF termination of electrodes 214 and 216 shown
in FIG. 2. Electrodes 314 and 316 are generally analogous to
electrodes 214 and 216 (FIG. 2). However, the sole purpose of the
RF termination provided by a load resistor R.sub.L connected to
electrodes 314 and 316 is to avoid signal reflections from the
right end of the electrodes. Hence, load resistor R.sub.L is not
connected to any external circuits and essentially is an internal
component of the corresponding Mach-Zehnder modulator. In contrast,
the right ends of electrodes 214 and 216 are RF terminated by being
connected to transmission line 240 that leads to the electrical
circuits that are external to Mach-Zehnder modulator 210, such as
ADC 250.
[0032] FIG. 4 shows a flowchart of an EPD method 400 that can be
used in transponder 100 (FIG. 1) according to an embodiment of the
disclosure. Method 400 can be run periodically or non-periodically,
e.g., as deemed necessary for maintaining an acceptable quality of
optical output signal 188 generated by transponder 100. In some
embodiments, method 400 may be advantageously combined with other
known or conventional EPD methods, e.g., directed at the
pre-compensation of signal distortions imposed in the optical
transport link, i.e., in the communication-system components
external to transponder 100. One of ordinary skill in the art will
appreciate that such external signal distortions may include but
are not limited to chromatic dispersion, polarization-mode
dispersion, and nonlinearly induced inter-symbol interference.
[0033] At step 402 of method 400, DSP 120 configures transmitter
170 to generate an optical output signal 188 carrying a training
data sequence. In some embodiments, the training data sequence may
be a part of a pilot signal or sequence used for other purposes,
such as frame synchronization, etc. In this case, the pilot signal
or sequence may be used synergistically for multiple purposes, one
of which includes an implementation of step 402 in method 400. In
alternative embodiments, the training data sequence may be
specifically configured to appropriately sample the full range of
signal distortions caused by transmitter 170.
[0034] At step 404, DSP 120 receives, via feedback paths 180, the
feedback signals 154 picked up, e.g., as illustrated in FIG. 2,
from one or more electrodes in optical modulator 176 during the
transmission of the training data sequence of step 402. In some
embodiments, DSP 120 may be optionally configured to store a copy
of the received feedback signals 154 in a memory (not explicitly
shown in FIG. 1), e.g., for further use at step 406.
[0035] At step 406, DSP 120 analyzes the feedback signals 154
received at step 404 to determine a set of parameter values for a
pre-distortion function to be applied to the waveforms
corresponding to payload data 112 in the process of generating
electrical digital signals 124. In an example embodiment, the
analysis may include a step of comparing the waveforms provided by
feedback signals 154 with a waveform template. The waveform
template may include a set of waveforms that are expected or
desired when transmitter 170 operates in an optimal regime while
transmitting the training data sequence. Based on this comparison,
DSP 120 determines the parameter values for the pre-distortion
function that tend to minimize or at least reduce to an acceptable
level the differences between the waveform template and a signal
that is generated by convolving the feedback signal(s) with the
pre-distortion function.
[0036] At step 408, DSP 120 configures the pre-distortion function
using the parameter values determined at step 406. In an example
embodiment, the application of the pre-distortion function to data
stream 112 in DSP 120 can be accomplished using a digital filter,
such as a finite-impulse-response (FIR) filter. Both time-domain
and frequency-domain implementations of the FIR filter are
contemplated. In this case, the parameter values of the
pre-distortion function that are set at step 408 may include a set
of tap coefficients of the FIR filter or frequency-domain
equivalents thereof. In an alternative embodiment, the application
of the pre-distortion function to data stream 112 in DSP 120 can be
accomplished using a look-up table (LUT). In the latter case, the
parameter values of the pre-distortion function that are set at
step 408 may include pattern-dependent correction information
extracted at step 406 and stored in the LUT for further use during
the payload-data transmission of step 410. In some embodiments, the
pre-distortion function may be configured using the appropriate
Volterra series, e.g., as disclosed in the following publications:
(i) Fadhel M. Ghannouchi and Oualid Hammi, "Behavioral Modeling and
Predistortion," IEEE Microwave Magazine, December 2009; and (ii)
Dennis R. Morgan, Zhengxiang Ma, Jaehyeong Kim, et al., "A
Generalized Memory Polynomial Model for Digital Predistortion of RF
Power Amplifiers," IEEE Transactions on Signal Processing--TSP,
2006, vol. 54, no. 10, pp. 3852-3860, both of which are
incorporated herein by reference in their entirety.
[0037] At step 410, DSP 120 uses the pre-distortion function
configured at step 408 to perform the EPD, in real time, on data
stream 112. Due to the EPD performed in DSP 120, transmitter 170 is
advantageously capable of causing optical output signal 188 to
carry the payload data of data stream 112 using desired or optimal
optical waveforms.
[0038] FIG. 5 graphically illustrates the use of all-electronic
feedback paths 180 (FIG. 1) in EPD method 400 (FIG. 4) according to
an embodiment of the disclosure. More specifically, a curve 502
graphically shows a frequency response (signal-transfer function)
of modulator 210 measured at an optical output thereof, such as an
output waveguide 260 (see FIG. 2). A curve 504 similarly
graphically shows a frequency response of modulator 210 measured at
the end of transmission line 240 (FIG. 2). A curve 506 graphically
shows an example desired effective frequency response of modulator
210 at output waveguide 260. One purpose of EPD method 400 may be
to configure and apply the pre-distortion function that causes the
hardware-imposed signal-transfer function 502 to be transformed
into the effective signal-transfer function 506. Example rationales
for such a transformation and alternative desired effective
frequency responses of modulator 210 are disclosed and explained,
e.g., in U.S. Patent Application Publication No. 2014/0029957,
which is incorporated herein by reference in its entirety.
[0039] The shape of the hardware-imposed signal-transfer function
502 generally varies among different nominally identical physical
copies of the same model of circuit 200 (FIG. 2) or transmitter 170
(FIG. 1). In addition, the shape of the hardware-imposed
signal-transfer function 502 may change over time, e.g., due to the
device aging, etc. As already indicated above, in situ measurements
of the hardware-imposed signal-transfer function 502 generally
require tapping the optical output signal generated by the
transmitter and then using a dedicated optical receiver coupled to
the optical tap for the detection of the tapped optical signal. As
such, the in situ measurement capability may be relatively
expensive to implement and not always available in the
corresponding optical transponder.
[0040] Various embodiments of transponder 100 disclosed herein at
least partially overcome these and some other related problems, in
effect, by using the electrically picked-up and measured frequency
response analogous to frequency response 504 as a proxy for the
hardware-imposed signal-transfer function 502, which is not
directly measured in transponder 100. Comparison of curves 502 and
504 reveals that, although curves 502 and 504 have different
general slopes (or tilts) at frequencies higher than about 5 GHz,
curve 504 faithfully reproduces many of the features of the fine
structure of curve 502. Note that the wiggles in both curves are
not noise, but rather are reproducible attributes of the hardware
performance. Further note the similarities in the wiggle patterns
in curves 502 and 504. These similarities enable EPD method 400 to
arrive at the pre-distortion function that causes the actually
achieved frequency response to closely approximate the desired
frequency response, such as that graphically shown by curve 506. As
such, EPD method 400 in effect starts from curve 504, as opposed to
starting from curve 502, to arrive at curve 506. Since the in situ
measurement of curve 504 does not require an optical tap and a
dedicated optical receiver, the cost of the EPD functionality in
transponder 100 may be significantly lower than the cost of the EPD
functionality in a comparably performing optical transponder, such
as a transponder that relies exclusively on optical feedback for
properly configuring its pre-distortion function.
[0041] In some embodiments, EPD method 400 may be modified to
include the steps of (i) estimating curve 502 based on curve 504
and (ii) deriving the parameters of and configuring the
pre-distortion function based on the estimated curve 502. As
already indicated above, the actual curve 502 is not measured in
transponder 100. However, the general slope difference between
curves 502 and 504 can be derived sufficiently accurately by
modeling the electro-optical signal conversion performed in the
corresponding optical modulator. The results of the modeling can be
incorporated, e.g., as a tilt-compensating template, into the
software or firmware used in DSP 120. An application of this
tilt-compensating template to the actually measured curve 504 then
produces a relatively accurate estimate of curve 502, wherein the
tilt-compensating template causes the estimated curve 502 to have
an approximately correct slope (tilt), while the fine structure of
the measured curve 504 supplies an approximately correct fine
structure for the estimated curve 502. The resulting estimated
curve 502 can then be used in the above-mentioned step (ii) in lieu
of the actual (unmeasured) curve 502 to arrive at the requisite
pre-distortion function using any of the suitable methods developed
for deriving the pre-distortion function based on the optical
feedback obtained by tapping the modulator's optical output.
[0042] FIG. 6 shows a block diagram of an optical transponder 600
according to an alternative embodiment of the disclosure.
Transponder 600 is generally similar to transponder 100 (FIG. 1)
and, as such, reuses many of the same components. The description
of the reused components is not repeated here. Rather, the
description of transponder 600 herein below focuses mainly on the
new components and differences with transponder 100.
[0043] One difference between transponders 100 and 600 is that the
latter has a plurality of partially optical feedback paths 680
(labeled 680.sub.1-680.sub.m, where m is a positive integer greater
than one). For example, a j-th feedback path 680.sub.j includes an
optical tap 604.sub.j configured to optically tap optical output
signal 188 and direct the tapped portion of that signal to a
photo-detector (e.g., photodiode) 610.sub.j. Photo-detector
610.sub.j operates to convert the optical signal received from
optical tap 604.sub.j into a corresponding electrical signal and
then apply this electrical signal to an electrical conductor
connected to an input port of one of (optional) electrical
amplifiers 148. A corresponding amplified electrical signal
generated by that electrical amplifier 148 is then converted into
digital form in the corresponding one of ADCs 150 to generate a
respective one of additional feedback signals 154. Note that
feedback paths 680 do not qualify as all-electronic feedback pats,
e.g., because each of them includes an O/E converter embodied by
the respective one of photo-detectors 610.
[0044] As the labeling in FIG. 6 implies, transponder 600 uses n+m
feedback signals 154, compared to just n feedback signals 154 in
transponder 100. This difference causes transponder 600 to also
employ more electrical amplifiers 148 and ADCs 150 than transponder
100. In some embodiments, transponder 600 may also incorporate an
optional semiconductor optical amplifier (SOA) 602 coupled to the
output of optical modulator 176 as indicated in FIG. 6.
[0045] In an example embodiment, feedback paths 680 may be used to
at least partially sample one or more frequency responses
(signal-transfer functions) of transmitter 170 exemplified by curve
502 shown in FIG. 5. This sampling can be used in DSP 120, e.g., to
improve the accuracy of approximating a desired frequency response
(e.g., curve 506 in FIG. 5). One of ordinary skill in the art will
appreciate that the accuracy of the approximation may be improved
because the set of parameters for the pre-distortion function can
now be determined more accurately based on the additional
hardware-performance information corresponding to curve 502, as
opposed to based on the hardware-performance information
corresponding only to curve 504, as in transponder 100.
[0046] In embodiments that include SOA 602, feedback paths 680 may
further be used to enable DSP 120 to pre-compensate for linear
and/or non-linear signal distortions imposed by the SOA.
[0047] In some embodiments, suitable coherent optical detectors can
be used to replace some or all of photo-detectors 610. In one
embodiment, each of these coherent optical detectors may include an
individual local-oscillator source similar to local oscillator
source 164 used in optical receiver 160. In an alternative
embodiment, each of these coherent optical detectors may be coupled
to receive a portion of optical beam 174 for use therein as a
local-oscillator signal. One of ordinary skill in the art will
appreciate that the use of coherent optical detectors instead of
photo-detectors 610 may further improve the accuracy of
approximating a desired frequency response.
[0048] Although various embodiments have been described in
reference to circuit 200 (FIG. 2) having only two electrodes in
modulator 210, other electrode and electrical-signal pick-up
configurations are also possible. For example, U.S. patent
application Ser. No. 14/202,703, filed on Mar. 10, 2014, discloses
optical modulators having three or more electrodes that may also be
used in various embodiments of optical modulator 176. Using the
above-provided description, one of ordinary skill in the art will
understand how to connect such multiple electrodes to
all-electronic feedback paths similar to feedback paths 180, e.g.,
to enable the generation of more than one feedback signal 154 per
Mach-Zehnder modulator. U.S. patent application Ser. No. 14/202,703
is incorporated herein by reference in its entirety.
[0049] The use of alternative modulator structures, e.g., different
from the structure of Mach-Zehnder modulator 210, are also
contemplated.
[0050] According to an example embodiment disclosed above in
reference to FIGS. 1-6, provided is an apparatus (e.g., 100, FIG.
1; 600, FIG. 6) comprising: an optical transmitter (e.g., 170, FIG.
1) configured to generate a modulated optical signal (e.g., 188,
FIG. 1) based on an electrical digital signal (e.g., 124, FIG. 1);
a digital signal processor (e.g., 120, FIG. 1) configured to apply
electronic pre-distortion (e.g., according to 400, FIG. 4) to
generate the electrical digital signal in a manner that reduces an
amount of signal distortions in the modulated optical signal caused
by hardware (e.g., 146, 176, 602; FIG. 6) of the optical
transmitter; and a first all-electronic feedback path (e.g.,
180.sub.1, FIG. 1) between the optical transmitter and the digital
signal processor configured to provide a first feedback signal
(e.g., one of 154, FIG. 1) for the digital signal processor to
determine one or more parameter values for the electronic
pre-distortion.
[0051] In some embodiments of the above apparatus, the electronic
pre-distortion is configured to cause an effective signal-transfer
function (e.g., 506, FIG. 5) exhibited by the optical transmitter
to differ from a signal-transfer function (e.g., 502, FIG. 5)
imposed by the hardware of the optical transmitter. As used herein,
the term "effective signal-transfer function" refers to the
signal-transfer function exhibited by the optical transmitter due
to the convolution, in the chain of signal processing, of the EPD
function imposed by the DSP and the signal-transfer function
imposed by the hardware of the optical transmitter. The effective
signal-transfer function may be linear or nonlinear.
[0052] In some embodiments of any of the above apparatus, the
apparatus further comprises a second all-electronic feedback path
(e.g., 180.sub.n, FIG. 1) between the optical transmitter and the
digital signal processor configured to provide a second feedback
signal (e.g., another one of 154, FIG. 1) for the digital signal
processor to determine the one or more parameter values for the
electronic pre-distortion.
[0053] In some embodiments of any of the above apparatus, the
apparatus further comprises a first partially optical feedback path
(e.g., 680.sub.1, FIG. 6) between the optical transmitter and the
digital signal processor configured to provide a third feedback
signal (e.g., yet another one of 154, FIG. 6) for the digital
signal processor to determine the one or more parameter values for
the electronic pre-distortion.
[0054] In some embodiments of any of the above apparatus, the
apparatus further comprises a second partially optical feedback
path (e.g., 680.sub.m, FIG. 6) between the optical transmitter and
the digital signal processor configured to provide a fourth
feedback signal (e.g., yet another one of 154, FIG. 6) for the
digital signal processor to determine the one or more parameter
values for the electronic pre-distortion.
[0055] In some embodiments of any of the above apparatus, the
apparatus further comprises a first partially optical feedback path
(e.g., 680.sub.1, FIG. 6) between the optical transmitter and the
digital signal processor configured to provide a second feedback
signal (e.g., another one of 154, FIG. 6) for the digital signal
processor to determine the one or more parameter values for the
electronic pre-distortion.
[0056] In some embodiments of any of the above apparatus, the first
partially optical feedback path comprises: an optical tap (e.g.,
604.sub.1, FIG. 6) configured to tap out a portion of an optical
power of the modulated optical signal; an optical-to-electrical
converter (e.g., 610.sub.1, FIG. 6) configured to convert the
portion of the optical power of the modulated optical signal into a
corresponding electrical signal; and an electrical conductor (e.g.,
the electrical line connecting 610.sub.1 and 148, FIG. 6)
configured to direct the corresponding electrical signal to an
input interface of the digital signal processor to provide thereat
the second feedback signal.
[0057] In some embodiments of any of the above apparatus, the
optical-to-electrical converter comprises a photodiode (e.g.,
610.sub.1, FIG. 6).
[0058] In some embodiments of any of the above apparatus, the
optical-to-electrical converter comprises a coherent optical
detector coupled to an optical local-oscillator source (e.g., 164,
FIG. 6).
[0059] In some embodiments of any of the above apparatus, the
optical transmitter comprises an optical modulator (e.g., 210, FIG.
2).
[0060] In some embodiments of any of the above apparatus, the first
all-electronic feedback path comprises: an electrical pick-up
terminal located in an electrical-circuit portion of the optical
modulator; and an electrical conductor (e.g., 240, FIG. 2)
configured to direct an electrical signal picked-up by the
electrical pick-up terminal to an input interface of the digital
signal processor to provide thereat the first feedback signal.
[0061] In some embodiments of any of the above apparatus, the
optical modulator comprises: an optical waveguide (e.g., 212.sub.1,
FIG. 2); a first electrode (e.g., 214, FIG. 2) positioned along a
length of the optical waveguide and having a first end (e.g., the
left end in FIG. 2) and a second end (e.g., the right end in FIG.
2), wherein: the first end is configured to receive an electrical
drive signal (e.g., 218, FIG. 2) generated based on the electrical
digital signal; and the electrical pick-up terminal (e.g., end of
240, FIG. 2) is electrically connected to the second end.
[0062] In some embodiments of any of the above apparatus, the
optical transmitter is configured to generate the electrical drive
signal in a manner that causes the first electrode to operate as a
traveling-wave electrode.
[0063] In some embodiments of any of the above apparatus, the
apparatus further comprises a second all-electronic feedback path
(e.g., 180.sub.n, FIG. 1) between the optical transmitter and the
digital signal processor configured to provide a second feedback
signal (e.g., another one of 154, FIG. 1) for the digital signal
processor to determine the one or more parameter values for the
electronic pre-distortion, wherein the second all-electronic
feedback path comprises: an additional electrical pick-up terminal;
and an additional electrical conductor configured to direct an
electrical signal picked-up by the additional electrical pick-up
terminal to the input interface of the digital signal processor to
provide thereat the second feedback signal.
[0064] In some embodiments of any of the above apparatus, the
optical modulator further comprises a second electrode (e.g., as
described in U.S. patent application Ser. No. 14/202,703)
positioned along a length of the optical waveguide.
[0065] In some embodiments of any of the above apparatus, the
additional electrical pick-up terminal is electrically connected to
the second electrode.
[0066] In some embodiments of any of the above apparatus, the
optical transmitter comprises an additional optical modulator
(e.g., another physical copy of 210, FIG. 2); the additional
optical modulator comprises a second electrode (e.g., another
physical copy of 214, FIG. 2) positioned along a length of another
optical waveguide; and the additional electrical pick-up terminal
is electrically connected to the second electrode.
[0067] In some embodiments of any of the above apparatus, the
optical modulator and the additional optical modulator are
optically coupled to one another by at least one optical
waveguide.
[0068] In some embodiments of any of the above apparatus, the first
all-electronic feedback path comprises an analog-to-digital
converter (e.g., 150, FIG. 1).
[0069] In some embodiments of any of the above apparatus, the first
all-electronic feedback path further comprises an electrical
amplifier (e.g., 148, FIG. 1).
[0070] According to another example embodiment disclosed above in
reference to FIGS. 1-6, provided is an electronic pre-distortion
method (e.g., 400, FIG. 4) comprising the steps of: (A) configuring
(e.g., 402, FIG. 4) an optical transmitter (e.g., 170, FIG. 1) to
generate an optical output signal (e.g., 188, FIG. 1) carrying a
training data sequence; (B) receiving (e.g., 404, FIG. 4), via a
first all-electronic feedback path (e.g., 180.sub.1, FIG. 1), a
first feedback signal (e.g., one of 154, FIG. 1) corresponding to
the optical output signal, wherein the first all-electronic
feedback path is configured to electrically connect the optical
transmitter and a digital signal processor (e.g., 120, FIG. 1); (C)
determining (e.g., 406, FIG. 4) one or more parameter values for
electronic pre-distortion based on the training data sequence and
the first feedback signal; and (D) configuring (e.g., 410, FIG. 4)
the optical transmitter to generate an optical signal carrying
payload data (e.g., 112, FIG. 1) with the digital signal processor
being configured (e.g., 408, FIG. 4) to apply the electronic
pre-distortion to the payload data using the one or more parameter
values.
[0071] In some embodiments of the above method, the method further
comprises the step of (E) receiving, via a second all-electronic
feedback path (e.g., 180.sub.n, FIG. 1), a second feedback signal
(e.g., another one of 154, FIG. 1), wherein the second
all-electronic feedback path is configured to electrically connect
the optical transmitter and the digital signal processor, wherein
step (C) comprises determining the one or more parameter values for
the electronic pre-distortion based on the second feedback
signal.
[0072] In some embodiments of any of the above methods, the method
further comprises receiving, via a first partially optical feedback
path (e.g., 680.sub.1, FIG. 6), a third feedback signal (e.g., yet
another one of 154, FIG. 6), wherein the first partially optical
feedback path is configured to electrically connect the optical
transmitter and the digital signal processor; and step (C)
comprises determining the one or more parameter values for the
electronic pre-distortion based on the third feedback signal.
[0073] In some embodiments of any of the above methods, the step of
determining comprises: estimating, based on first feedback signal,
a frequency response (e.g., 502, FIG. 5) of the optical transmitter
that would have been measured at an optical output thereof; and
determining the one or more parameter values for electronic
pre-distortion based on the estimated frequency response.
[0074] While this disclosure includes references to illustrative
embodiments, this specification is not intended to be construed in
a limiting sense. Various modifications of the described
embodiments, as well as other embodiments within the scope of the
disclosure, which are apparent to persons skilled in the art to
which the disclosure pertains are deemed to lie within the
principle and scope of the disclosure, e.g., as expressed in the
following claims.
[0075] Unless explicitly stated otherwise, each numerical value and
range should be interpreted as being approximate as if the word
"about" or "approximately" preceded the value of the value or
range.
[0076] It will be further understood that various changes in the
details, materials, and arrangements of the parts which have been
described and illustrated in order to explain various embodiments
may be made by those skilled in the art without departing from the
scope of the invention as expressed in the following claims.
[0077] Although the elements in the following method claims, if
any, are recited in a particular sequence with corresponding
labeling, unless the claim recitations otherwise imply a particular
sequence for implementing some or all of those elements, those
elements are not necessarily intended to be limited to being
implemented in that particular sequence.
[0078] Reference herein to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment can be included in at
least one embodiment of the invention. The appearances of the
phrase "in one embodiment" in various places in the specification
are not necessarily all referring to the same embodiment, nor are
separate or alternative embodiments necessarily mutually exclusive
of other embodiments. The same applies to the term
"implementation."
[0079] Also for purposes of this description, the terms "couple,"
"coupling," "coupled," "connect," "connecting," or "connected"
refer to any manner known in the art or later developed in which
energy is allowed to be transferred between two or more elements,
and the interposition of one or more additional elements is
contemplated, although not required. Conversely, the terms
"directly coupled," "directly connected," etc., imply the absence
of such additional elements.
[0080] The functions of the various elements shown in the figures,
including any functional blocks labeled as "processors," may be
provided through the use of dedicated hardware as well as hardware
capable of executing software in association with appropriate
software. When provided by a processor, the functions may be
provided by a single dedicated processor, by a single shared
processor, or by a plurality of individual processors, some of
which may be shared. Moreover, explicit use of the term "processor"
or "controller" should not be construed to refer exclusively to
hardware capable of executing software, and may implicitly include,
without limitation, digital signal processor (DSP) hardware,
network processor, application specific integrated circuit (ASIC),
field programmable gate array (FPGA), read only memory (ROM) for
storing software, random access memory (RAM), and non volatile
storage. Other hardware, conventional and/or custom, may also be
included. Similarly, any switches shown in the figures are
conceptual only. Their function may be carried out through the
operation of program logic, through dedicated logic, through the
interaction of program control and dedicated logic, or even
manually, the particular technique being selectable by the
implementer as more specifically understood from the context.
[0081] It should be appreciated by those of ordinary skill in the
art that any block diagrams herein represent conceptual views of
illustrative circuitry embodying the principles of the
invention.
[0082] The use of figure numbers and/or figure reference labels in
the claims is intended to identify one or more possible embodiments
of the claimed subject matter in order to facilitate the
interpretation of the claims. Such use is not to be construed as
necessarily limiting the scope of those claims to the embodiments
shown in the corresponding figures.
* * * * *